Digital communication systems modulate an electromagnetic carrier signal to convey binary information from a transmitter to a receiver. Two well-known digital modulation techniques are QAM (Quadrature Amplitude Modulation) and QPSK (Quadrature Phase Shift Keying). Both QAM and QPSK are forms of I/Q Modulation because both of these modulation techniques compose a signal from two independent components: an in-phase component (I) and a quadrature component (Q). Since the in-phase and quadrature components are orthogonal to each other, they do not interfere with each other during transmission. The I and Q components are represented graphically by plotting I and Q on a plane Cartesian coordinate system, known as a phasor diagram, in which the I value is conventionally the abscissa while the Q value is the ordinate.
Demodulation of the in-phase and quadrature components of the received signal recovers the I bits and Q bits, respectively. When plotted on the phasor diagram, the signals form an I/Q constellation, or a signal point constellation. For good reception, the signal points should cluster tightly around discrete points, or “nominal states”, which are then readily demodulated into binary 1's and binary 0's. When the signal points cluster tightly around the nominal states, the carrier is said to be locked.
A coherent receiver, such as an optical coherent receiver, must be locked, or “in lock”, to ensure that binary data is received at a minimal rate of error, usually expressed in terms of bit-error rate, or BER. Bit-error rate is customarily defined as the average probability that the receiver will incorrectly identify a bit. Raw BER is BER before decoding by FEC (forward error correction). With the use of powerful FEC, the BER can be reduced by many orders of magnitude compared to the raw BER.
A carrier can go “out of lock” when severe distortion occurs. For an optical coherent receiver, distortion may occur because of PMD (Polarization-Mode Dispersion), PDL (Polarization Dependent Loss), ASE (Amplified Spontaneous Emission) or any combination thereof.
In communication systems using QAM or QPSK, carrier lock is necessary to permit accurate decoding of the signals and recovery of the baseband I and Q bits. In other words, to enable the modulated signals to be correctly decoded, the phase and frequency of the receiver LO (local oscillator) must be locked to the incoming carrier.
In microwave communication systems, fading is a common and well-known phenomenon. During a fade, the signal-to-noise ratio (SNR) of the received signal falls to a low level, such that the signal and/or carrier lock may be lost. At the end of a fade, carrier lock must first be established before the signal can be correctly decoded. A carrier lock detector is used to detect whether the carrier is locked or unlocked. The carrier lock detector enables the decoder only when carrier lock is detected.
It is desirable to maintain carrier lock, and to detect carrier lock with a high degree of reliability, at the lowest possible levels of SNR, so that the system can tolerate deep fades with minimal loss of decoded data.
U.S. Pat. No. 4,736,386 (Nichols) entitled CARRIER OUT-OF-LOCK DETECTOR APPARATUS teaches a carrier lock detector which is responsive to I and Q (in-phase and quadrature) amplitude error bits to provide an out-of-lock indication when, on average, more than half of the detected I and Q amplitudes are in error. During an out-of-lock condition the detected signal point positions rotate about the I and Q axes of the phase plane diagram, the rate of rotation of the detected signal point positions being dependent upon the carrier phase error.
A disadvantage of this known carrier lock detector is that, at low levels of SNR during fading, it can indicate an out-of-lock condition even though the carrier is still locked. For example, for a 512-QAM system such a known detector fails to operate correctly for a SNR of less than about 29 dB, whereas it is desirable to have a detector which operates correctly at lower levels of SNR, for example down to about 25 dB.
An improved carrier lock detector is disclosed in U.S. Pat. No. 4,987,375 entitled CARRIER LOCK DETECTOR FOR A QAM SYSTEM (Wu et al.), illustrated in FIG. 1. As shown in FIG. 1, the carrier lock detector includes a QAM demodulator 10, which splits the signal into analog I and Q components on respective output lines 14. A pair of analog-to-digital converters (ADC) 12 convert the analog I and Q components into I bits and Q bits on lines 16. The carrier lock detector further includes a gating circuit 18, an encoder 20, an integrating circuit 22, and a comparator 24. The gating circuit 18 has two EXCLUSIVE-OR gates 26 and 28 as well as two AND gates 30 and 32. The integrating circuit 22 includes an integrating capacitor 34 coupled via resistors 36 to the outputs of the encoder 20 and via resistors 38 to the differential inputs of a differential amplifier 40, having a negative feedback resistor 42 and a resistor 44 between its non-inverting input and ground, and having a smoothing circuit with a resistor 46 and a capacitor 48 coupled to its output. The resistors 42 and 44 have the same resistance, and the resistors 36, 38 each have half that resistance. The comparator 24 compares the smoothed output of the integrating circuit 22 with a reference voltage Vref to produce an output Vout which constitutes a carrier lock detection signal.
FIG. 2 shows an I/Q plot 50 of a prior-art lock detection algorithm implemented on a QPSK or 4-QAM system. Centered about four nominal states 52 are four first areas 54 defined by (I2⊕I3)·(Q2⊕Q3). Adjacent to the first areas are second areas 56 defined by I2⊕I3· Q2⊕Q3. If a detected signal has I and Q components that map onto one of the four areas 54 surrounding the nominal states 52, a first signal is generated. If the detected signal has I and Q components that map onto one of the second areas 56, then a second signal is generated. The difference between the first and second signals is compared with a threshold value. If the difference exceeds the threshold, then a carrier lock detection signal is generated.
Although the carrier lock detector described by Wu et al. worked well for large constellations, such as 16-QAM to 512-QAM, for lower-level QAM such as 4-QAM or for QPSK, the method and apparatus described by Wu et al. is not effective. In light of the very important optical applications such as 2-pol QPSK, due to the effect of laser linewidth and other distortions such as PMD and PDL and the use of powerful FEC, there is a need-for a much more robust carrier lock detector that can operate at raw BER of 1e-2 or higher. Furthermore, due to the high speed of optical modems (circa 10 GBaud), it is highly desirable that the hardware implementation of this carrier lock detector be simple.